public class habitatOrg25a {

public void invokeExternalWs()  
        {  
         
             String username = 'sbhargava';
             String password = 'Habitat12';
             String token = username+':'+password;
             
             Blob hash = Blob.valueOf(token);       
           
            HttpRequest hReq = new HttpRequest();
            hReq.setEndpoint('https://partnerwebtest.habitat.org:8443/PimsService.asmx');
            hReq.setMethod('POST');
            hReq.setClientCertificateName('PIMS_Certificate');
            
            
            hReq.setHeader('Authorization', 'Basic '+ EncodingUtil.base64Encode(hash));
            hReq.setHeader('Content-Type', 'text/xml');  
            hReq.setHeader('SOAPAction', 'https://partnerwebtest.habitat.org:8443/PimsService.asmx');//  
            //hReq.setBody(); 
           
            Http con = new Http();
      
            try {  
                
                HttpResponse res = con.send(hReq);
            
              system.debug('#### ' + res.getStatus()); 
              //Execute web service call here         
            //YOU CAN ALWAYS PARSE THE RESPONSE XML USING XmlStreamReader  CLASS  
           } catch(System.CalloutException e) {  
               System.debug('Exception:'+e);
         }         
    }

}